The major leftist labor federation in El Salvador, with ties to the rebel movement, quietly has been gaining strength and now is poised to become the country's largest urban union.

The federation's growth represents a potential tool for organizing a renewal of street agitation such as the protests that led to repeated bloodshed and brutality two years ago, analysts here warn. Street violence has all but disappeared since guerrilla tactics shifted to war in the countryside and leftist organizations in San Salvador were crushed by the U.S.-backed military.

The federation, the United Labor Movement of El Salvador, has found recruiting easier in recent months because restrictive wartime decrees that severely limit union activity have generated resentment against the government among workers whose pay and benefits have suffered.

"They know that the more workers are unable to conduct normal collective bargaining, the more they are willing to listen to these groups," a labor specialist said.

Particularly worrying to U.S. and Salvadoran officials is the reportedly imminent decision of the leading San Salvador municipal employes' union to join forces with the federation. Officials of the federation and the union, the General Association of Public and Municipal Employes, have said talks are under way and an agreement is expected soon.

The municipal group boasts 12,000 members. Its association with the labor federation will bring the leftist-controlled umbrella group's overall strength to nearly 50,000, according to reliable estimates.

This will make the federation more powerful in the cities, mainly San Salvador, than the Popular Democratic Unity, the centrist labor federation backed by the AFL-CIO. It also raises the possibility of a leftist-oriented labor federation exercising influence over municipal employes who could disrupt essential city services as part of any antigovernment agitation, they add.

The federation officials, with some of their groups banned under law and some of their leaders sought by security forces, emphasize purely labor goals and say they have no political ties to the guerrilla movement or its political arm, the Democratic Revolutionary Front. However, the federation's stands closely parallel those of the guerrilla movement, including rejection of U.S.-backed elections.

In a communique issued on May Day, the group declared:

"Salvadoran workers and people, we are living a difficult historic moment in which we will decide whether we continue under the chains of domination imposed by the traditional oligarchy with the unconditional approval of the Yankee government and expressed in the outmoded dictatorship that oppresses us, or whether we use the great combative force and ability that the workers and people possess to break the oppression and dependence that until today rule in our country."

The Popular Democratic Unity, which has ties to El Salvador's Christian Democratic Party, was created three years ago as a democratic counter to political forces that have since melded into the Democratic Revolutionary Front. It has been an important channel for U.S. influence among Salvadoran workers and peasants and, according to a recent study by researchers at the University of Central America, "is in large measure financed, organized and administered by the North American unions."

Popular Democratic Unity officers and their U.S. backers claim a membership of several hundred thousand. But even sympathetic labor analysts here dismiss the claim as exaggerated, putting the membership at a maximum of 100,000. Of that figure, they add, more than half comes from a peasant group called the Salvadoran Communal Union created by the AFL-CIO's American Institute of Free Labor Development.

The country's main rightist party, the National Republican Alliance led by former major Roberto d'Aubuisson, has sought since August 1982 to create a sympathetic labor federation of its own, the National Workers Confederation. Analysts noted a confederation communique in December endorsing worker rights and political moderation, but the group's low membership means it wields little practical influence.

Worker rights such as pensions and insurance are among the issues raised in the new constitution that are expected to generate hot debate in the constituent assembly. The assembly is now considering the document in a point-by-point vote that began last week.

"In other countries, these are already natural things, but here they are still revolutionary," said a Christian Democratic assembly member. Most of the opposition to labor reforms is expected to come from the rightist party members, he added.

Decrees imposed by the junta that ruled until April last year have laid down strict limits on labor organizations. One bars strikes by government employes, for example, and another forbids any union or trade association that threatens "the security of the state." A third, perhaps most resented by workers, freezes salaries except for government-set annual raises.